The present invention relates to a scintillator. More specifically, the present invention relates to a CeBr3 scintillator for use in gamma ray spectroscopy and x-ray detection.
Scintillation spectrometers are widely used in detection and spectroscopy of energetic photons (X-rays and γ-rays) at room temperature. These detectors are commonly used in nuclear and particle physics research, medical imaging, diffraction, non destructive testing, nuclear treaty verification and safeguards, nuclear non-proliferation monitoring, and geological exploration (Knoll Radiation Detection and Measurement, 3rd Edition, John Wiley and Sons, New York, (1999), Kleinknecht, Detectors for Particle Radiation, 2nd Edition, Cambridge University Press, Cambridge, U.K. (1998)).
Important requirements for the scintillation crystals used in these applications include high light output, high stopping efficiency, fast response, good proportionality, low cost and availability in large volume. These requirements cannot be met by any of the commercially available scintillators. As a result, there is continued interest in the search for new scintillators with enhanced performance (Derenzo, in, Heavy Scintillators for Scientific and Industrial Applications, De Notaristefani et al. eds., Gif-sur-Yvette, France (1993), pp. 125-135; van Eijk, Lecoq, Proc. Int. Conf. Inorganic Scintill. Appl., pps. 3-12, Shanghai, China, (1997); Moses, Nuc. Inst. Meth. A-487:123-128 (2002)).
One of the uses in geological evaluation includes well logging or formation evaluation. These are terms used for the measurement versus depth or time, or both, of one or more physical quantities in or around a well. Typically, a logging tool is lowered into a borehole and then retrieved from the hole while recording measurements. Wireline logs are taken “downhole”, transmitted through a wireline to the surface and recorded there. Measurement-while-drilling (MWD) and logging-while-drilling (LWD) measurements are also taken “downhole”. The measurements are either transmitted to the surface by mud pulses, or else recorded “downhole” and retrieved later when the instrument is brought to the surface. Mud logs that describe samples of drilled cuttings are taken and recorded at the surface.
Measurements typically taken during well logging or formation evaluation involve, for example, 1) natural gamma-ray spectroscopy to measure the spectrum or number and energy of gamma-rays emitted as natural radioactivity by a formation; 2) neutron activation which provides a log of elemental concentrations derived from the characteristic energy levels of gamma-rays emitted by a nucleus that has been activated by neutron bombardment; 3) epithermal neutron porosity measurement which is a measurement based on the slowing down of neutrons between a source and one or more detectors that measure neutrons at the epithermal level, where their energy is above that of the surrounding matter; 4) elastic neutron scattering which involves neutron interaction in which the kinetic energy lost by a neutron in a nuclear collision is transferred to the nucleus; and 5) gamma-ray scattering which is used for a measurement of the bulk density of a formation based on the reduction in gamma-ray flux between a source and a detector due to Compton scattering.
Scintillation and semiconductor detectors are typically used in these logging devices. It is well known that the static temperature in a wellbore increases gradually with depth. In most of North America the increase or “gradient” will be between 0.5 and 2.5° F. for each 100 feet of increase in depth, with a value of 1.7° F./100 feet (3° C./100 meters) being typical. For these applications, one of the important characteristics of the detector is its ability to perform at high temperatures. Typical scintillators used in well logging devices include BGO and CsI(Tl) which perform poorly as temperature increases, losing half of their light output at around 75° C. and 130° C., respectively. Furthermore, the variability of output with temperature of the scintillators necessitates careful calibration procedures.
In the present invention, properties of a new scintillator, cerium bromide (CeBr3), are disclosed. In this material, Ce3+ is an intrinsic constituent as well as a luminescence center for the scintillation process. The γ-ray stopping efficiency of CeBr3 is significantly higher than that of NaI(Tl), the most common scintillation detector. Small crystals of CeBr3 have been grown using the Bridgman process and their scintillation properties have been characterized. High light output, good proportionality, fast response and excellent energy and timing resolution have been measured for small CeBr3 crystals. In addition, CeBr3 was also found to possess excellent scintillation properties including high light output and fast response at high temperature (for example, at about 175° C.). Based on the results, CeBr3 was demonstrated to be very promising for γ-ray spectroscopy and its properties were determined to be very similar to those of another recently discovered scintillator, cerium doped lanthanum bromide (LaBr3:Ce) (van Loef et al., Appl. Phys. Lett. 79:1573 (2001)).
Attention is drawn to several references in the field, the teachings of which are incorporated herein by reference (as are all references cited herein):
Derenzo et al., Nucl. Inst. Meth. Physics Res. A 505:111-117 (2003), entitled “The Quest for the Ideal Scintillator” reviews the history of, and characteristics and mechanisms of many inorganic scintillators.
U.S. Pat. No. 5,319,203 and U.S. Pat. No. 5,134,293, both entitled “Scintillator material.” Discloses Cerium fluoride and thallium doped Cerium fluoride as “improved” scintillator material.
U.S. Pat. No. 5,039,858, “Divalent fluoride doped cerium fluoride scintillator.” Discloses additional doped cerium fluoride scintillators.
Moses et al., J. Luminescence 59:89-100 (1994), entitled “Scintillation Mechanisms in Cerium Fluoride” described studies of the scintillation mechanisms of cerium fluoride and of lanthanum fluoride doped with cerium in concentrations between 0.01% and 50% mole fraction cerium.
U.S. Pat. No. 4,510,394, “Material for scintillators.” Discloses barium fluoride as scintillator material.
van Loef et al., “High energy resolution scintillator: Ce3+ activated LaBr3”, Appl. Phys. Lett. 79:1573-1575 (2001).
van Loef et al., “Scintillation properties of LaBr3:Ce3+ crystals: fast, efficient and high-energy-resolution scintillators”, Nucl. Instr. Meth. Physics Res. A 486:254-258 (2002). Discloses certain characteristics of cerium doped LaBr3 compositions including, light yield, and scintillation decay curve. The rise time and time resolution of the compositions are not disclosed or suggested.
WO 01/60945, “Scintillator crystals, method for making same, use thereof”, Discloses inorganic scintillator material of the general composition M1−xCexBr3, where M is selected from lanthanides or lanthanide mixtures of the group consisting of La, Gd, and Y. X is the molar rate of substitution of M with cerium, x being present in an amount of not less than 0.01 mol % and strictly less than 100 mol %. The rise time and time resolution of the various compositions are not disclosed or suggested.
U.S. Pat. No. 6,362,479, “Scintillation detector array for encoding the energy, position, and time coordinates of gamma ray interactions,” discloses a scintillator-encoding scheme that depends on the differential decay time of various scintillators. The use of lutetium orthosilicate-lutetium orthosilicate (LSO-LSO) crystals with a time resolution of 1.6 ns is also discussed. A time resolution of 1.6 ns is equivalent to an approximately 50 cm uncertainty, which is as large as the cross-sectional dimension of the human body, and not useful in TOF-PET.
U.S. Pat. No. 5,453,623, “Positron emission tomography camera with quadrant-sharing photomultipliers and cross-coupled scintillating crystals.” Discloses arrangement of hardware elements in PET camera and use of scintillators. Only specific scintillator disclosed is BGO.
Moses et al., “Prospects for Time-of-Flight PET using LSO Scintillator,” IEEE Trans. Nucl. Sci. 46:474-478 (1999). Discloses measurements of the timing properties of lutetium orthosilicate (LSO) scintillator crystals coupled to a PMT and excited by 511 keV photons.